This invention relates to a process for making encapsulated liquid crystal material suitable for use in electrooptical devices.
Many types of liquid crystal devices are known. Among these the most well known are displays, but other liquid crystal devices include privacy screens, sunroofs, membrane switches, and shutters. A preferred type of liquid crystal device employs encapsulated liquid crystal material, wherein liquid crystals are encapsulated or dispersed in a matrix or containment medium such as a polymer. When a voltage corresponding to a sufficiently strong electric field is applied across the encapsulated liquid crystal material (the "field-on" condition), the alignment of the liquid crystals is re-oriented in accordance with the field, so that incident light is transmitted. Conversely, in the absence of such a voltage (the "field-off" condition) the alignment of the liquid crystals is random and/or influenced by the liquid crystal-matrix interface, so that the liquid crystal material scatters incident light. The applied voltage at which the liquid crystal material begins to change from its field-off condition to its field-on condition is called the threshold voltage. Encapsulated liquid crystal materials and their use in devices are discussed in Fergason, U.S. Pat. Nos. 4,435,047 (1984), 4,579,423 (1986), 4,605,284, 4,616,903, and 4,707,080; Doane et al., U.S. Pat. No. 4,890,902; West et al., U.S. Pat. No. 4,685,771 (1987); and Doane et al., U.S. Pat. No. 4,688,900 (1987), the disclosures of which are incorporated herein by reference.
The size and size distribution of the liquid crystals droplets contained in the matrix can affect the performance of the liquid crystal material. When an encapulated liquid crystal material is prepared, the droplets will be produced in a range of sizes. The smaller droplets have a higher threshold voltage, so that in a material having a substantial amount of smaller, submicron sized droplets, or fines, switching all the liquid crystal droplets to the field-on condition requires a higher voltage. The result is that material having a large amount of fines will appear hazy until such higher voltage is applied, and will not switch sharply between a nontransmissive state and a transmissive state. An encapsulated liquid crystal material in which the droplet size distribution is relatively narrow will also have a sharper turn-on effect, because more of the droplets will have the same threshold voltage. It is also taught in Wu et al., U.S. Pat. No. 4,671,618 (1987) that the switching time of encapsulated liquid crystal material is affected by the droplet size.
In one method of making encapsulated liquid crystal material, an emulsion of the containment medium and liquid crystals is initially produced, optionally together with a carrier medium. The use of water as a carrier medium is taught in the aforementioned Fergason U.S. Pat. No. 4,435,047. The emulsion is spread onto a substrate and allowed to dry, to produce a film or sheet of encapsulated liquid crystal material. It is desirable that the emulsion be readily spreadable onto the substrate, to form uniform and defect-free films. Further, it is also desirable that the carrier medium be readily volatilized, so that the films dry quickly and are less likely to incur defects or be contaminated. Otherwise, there may be redistribution of the droplets, leading to an inhomogeneous dried film, due to variations in the flatness and/or surface energy of the substrate.